UNIVERSITY   OF  CALIFORNIA  PUBLICATIONS 

COLLEGE  OF  AGRICULTURE 

AGRICULTURAL  EXPERIMENT  STATION 

BERKELEY,  CALIFORNIA 


THE  COMMON  HONEY  BEE  AS  AN 
AGENT  IN  PRUNE  POLLINATION 

Second  Eeport 


BY 
A.  H.  HENDRICKSON 


BULLETIN  No.  291 

January,  1918 


UNIVERSITY  OF  CALIFORNIA  PRFSS 

BERKELEY 

1918 


Benjamin  Ide  Wheeler,  President  of  the  University. 

EXPEEIMENT  STATION  STAFF 

HEADS   OF  DIVISIONS 

Thomas  Forsyth  Hunt,  Director. 

Edward  J.  Wickson,  Horticulture    (Emeritus). 

Herbert  J.  Webber,  Director  Citrus  Experiment  Station;   Plant   Hrocdiny 

Hubert  E.  Van  Norman,  Vice-Director;  Dairy  Management. 

William  A.   Setchell,  Botany. 

Myer  E.  Jaffa,  Nutrition. 
*Eobert  H.  Loughridge,  Soil  Chemistry  and  Physics  (Emeritus) 

Charles  W.  Woodworth,  Entomology. 

Ealph  E.  Smith,  Plant  Pathology. 

J.  Eliot  Coit,  Citriculture. 

John  W.  Gilmore,  Agronomy. 

Charles  F.  Shaw,  Soil  Technology. 

John  W.  Gregg,  Landscape  Gardening  and  Floriculture. 

Frederic  T.  Bioletti,  Viticulture  and  Enology. 

Warren  T.  Clarke,  Agricultural  Extension. 

John  S.  Burd,  Agricultural  Chemistry. 

Charles  B.  Lipman,  Soil  Chemistry  and  Bacteriology. 

Clarence  M.  Haring,  Veterinary  Science  and  Bacteriology 

Ernest  B.  Babcock,  Genetics. 

Gordon  H.  True,  Animal  Husbandry. 

James  T.  Barrett,  Plant  Pathology. 

Fritz  W.  Woll,  Animal  Nutrition. 

Walter  Mulford,  Forestry. 

W.  P.  Kelley,  Agricultural  Chemistry. 

H.  J.  Quayle,  Entomology. 

Elwood  Mead,  Eural  Institutions. 

J.  B.  Davidson,  Agricultural  Engineering. 

H.  S.  Eeed,  Plant  Physiology. 

D.  T.  Mason,  Forestry. 

C.  L.  Eoadhouse,  Dairy  Industry. 
fFRANK  Adams,  Irrigation  Investigations. 

W.  L.  Howard,  Pomology. 

O.  J.  Kern,  Agricultural  Education. 

John  E.  Dougherty,  Poultry  Husbandry. 

S.  S.  Eogers,  Olericulture. 

David  N.  Morgan,  Assistant  to  the  Director. 

Mrs.  D.  L.  Bunnell,  Librarian. 

Division  of  Pomology 

W.  L.  Howai  d  W.  P.  Tufts 

E.  H.  Taylor  Eichard  Schmidt 

W.  L.  Sweet  Guy  L.  Philp 

A.  H.  Hendrickson  Milo  Wood 


*  Died  July  1,  1917. 

t  In  co-operation  with  office  of  Public  Eoads  and  Eural  Engineering,  U.   S. 
Department  of  Agriculture. 


THE  COMMON  HONEY  BEE  AS  AN  AGENT  IN 

PRUNE  POLLINATION 


BY 

A.  H.  HENDRICKSON 


INTRODUCTION 

A  second  report  on  the  part  played  by  the  honey  bee  in  the 
pollination  of  prunes  in  the  Santa  Clara  Valley  is  given  herewith. 
The  first  report1  showed  conclusively  that  bees  are  a  necessary  aid 
to  pollination  with  the  French  and  Imperial  varieties  of  prunes.  It 
also  showed  that  while  one  or  both  varieties,  but  particularly  the 
former,  may  set  a  fair  crop  when  but  few  bees  are  present,  they 
produce  a  heavier  crop  when  bees  are  numerous  in  the  orchard 
throughout  the  blossoming  season.  It  further  showed  that  prac- 
tically no  fruit  was  produced  when  all  pollen-carrying  insects  were 
kept  from  visiting  the  blossoms.  At  the  same  time  it  raised  the 
question  why  the  Imperial  prune  tree  did  not  respond  to  pollination 
by  bees  in  a  manner  similar  to  the  response  of  the  French  tree.  It 
left  unsolved,  however,  the  question  as  to  whether  the  French  or 
the  Imperial  prune  could  set  a  crop  of  fruit,  when  worked  upon  by 
bees  which  were  not  allowed  to  come  into  contact  with  any  other 
variety. 

PLAN    OF   SECOND   YEAR'S    WORK 

The  work  with  the  honey  bees  was  done  in  connection  with  a  large 
pollination  project  being  carried  on  by  the  Division  of  Pomology  of 
the  University  of  California.  As  in  the  case  of  the  previous  year, 
the  experiment  was  performed  on  the  Sorosis  Ranch,  situated  in 
Santa  Clara  County,  about  three  miles  northeast  of  Saratoga  on 
Saratoga  Avenue.  The  orchard  is  made  up  principally  of  French 
(Agen)  prunes,  eighteen  to  twenty-five  years  old.  Four  rows  of 
Imperials  topworked  on  Robe  de  Sergeant  run  cross-ways  of  the  part 
of  the  orchard  situated  on  the  north  side  of  the  road.  The  trees  are 
on  mixed  rootstocks ;  some  on  almond,  some  on  peach,  and  others  on 
Myrobalan.  The  soil  is  a  well-drained  clay  loam,  inclined  to  be  some- 
what gravelly  in  places,  particularly  in  the  section  occupied  by  the 
Imperials.  The  general  condition  of  the  trees,  with  the  possible 
exception  of  some  of  the  Imperials,  is  better  than  the  average  in  the 

i  Cal.  Exp.  Sta.  Bui.  274. 


216 


UNIVERSITY   OF    CALIFORNIA EXPERIMENT   STATION 


district.  A  cover  crop  of  barley  and  vetch  is  plowed  under  every 
spring,  and  barnyard  manure  from  the  dairy,  run  in  connection  with 
the  ranch,  is  applied  liberally  every  two  or  three  years.  The  trees  are 
given  a  regular  dormant  spray  and  pruned  lightly  each  year. 

The  experiment  was  conducted  by  erecting  tents  of  white  mosquito 
bar  over  the  experimental  trees  so  that  bees  and  other  pollen-carrying 
insects  could  be  excluded  or  confined.  The  experiment  of  1916,  in 
which  one  pair  of  adjoining  French  and  Imperial  prune  trees  were 
confined  in  a  tent  with  a  hive  of  bees,  and  one  pair  of  trees  of  the 


Fig.  1. — Tent  of  white  mosquito  bar  erected  over  a  pair  of  adjoining  French  and 

Imperial  prune  trees. 


same  varieties  kept  free  from  all  bees,  was  repeated.  In  addition, 
one  French  tree  and  one  Imperial  tree,  respectively,  were  encased 
in  single  tents  with  bees.  All  of  the  trees  used  were  as  nearly  the 
same  size  and  in  the  same  condition  of  health  and  vigor  as  it  was 
possible  to  secure  them.  The  double  tents  over  the  pairs  of  adjoining 
trees  were  about  40  feet  long,  25  feet  wide,  and  15  feet  high  (Fig.  1). 
The  single  tents  were  16  feet  long,  16  feet  wide,  and  15  feet  high 
(Fig.  2).      They  were  as  nearly  insect-proof  as  possible. 

Three  trees  each  of  the  French  and  Imperial  varieties  were  used. 
Two  double  tents  were  erected,  each  enclosing  a  pair  of  adjoining 


THE   COMMON   HONEY  BEE   IN  PRUNE  POLLINATION 


217 


French  and  Imperial  trees.  Single  tents  housed  the  two  remaining 
trees,  a  French  and  an  Imperial,  nsed  in  the  experiment.  Thus,  four 
tents  were  erected  to  enclose  the  six  trees  grouped  in  the  combinations 
mentioned.  In  one  tent,  enclosing  a  pair  of  French  and  Imperial 
trees,  a  hive  of  bees  was  kept  for  six  days  during  the  blossoming 
period  (Fig.  3).  All  pollen-carrying  insects  were  excluded  from 
the  second  double  tent  which  enclosed  the  other  pair  of  French  and 
Imperial  trees.      A  hive  of  bees  was  kept  in  each  of  the  single  tents, 


Fig.  2. — Tent  of  white  mosquito  bar  erected  over  a  single  tree. 


which  enclosed  a  French  and  an  Imperial  tree,  respectively.  It  was 
thought  that  practically  all  of  the  questions  arising  from  the  use 
of  bees  in  an  orchard  where  these  two  varieties  of  prunes  were 
planted  would  in  this  way  be  tested.  Under  the  tent  in  which  there 
were  no  bees,  the  effect  of  the  absence  of  pollen-carrying  agents  was 
tested.  In  the  tent  enclosing  both  a  French  and  an  Imperial  tree, 
the  question  of  inter-fertility  or  the  necessity  for  interplanting  for 
purposes  of  cross-pollination  between  the  two  varieties  was  studied. 
In  the  single  tents  the  ability  of  each  variety  to  set  fruit  with  its. 
own  pollen  was  tried  out.      In  addition  to  the  experiments  outlined 


218 


UNIVERSITY   OF    CALIFORNIA EXPERIMENT   STATION 


above,  counts  were  made  of  blossoms  of  each  variety  on  trees  growing 
in  the  open,  in  order  to  get  the  set  of  fruit  that  occurred  under  aver- 
age orchard  conditions  (Fig.  4).  The  last  named  count  served  as  a 
check  on  the  experiments. 


Fig.  3. — Bee  hive  under  tent  enclosing  a  prune  tree.      A  cluster  of  bees  may  be 

seen  at  the  entrance  of  the  hive. 


The  trees  which  had  been  used  in  the  experiment  during  the  pre- 
vious season  were  also  carefully  watched.  In  order  to  study  the 
effects  of  the  abnormal  crops  produced  the  previous  season,  counts 
of  the  blossoms  were  made  to  find  out  the  amount  of  fruit  set,  and 
observations  were  also  kept  of  the  general  growth  of  the  trees. 


THE   COMMON  HONEY   BEE  IN  PRUNE  POLLINATION 


219 


In  addition  to  the  tests  outlined,  other  orchards  where  bees  were 
kept  among  the  trees  in  1917  were  under  close  observation.      Follow 


iip*  i  ... 


I 


£ 


"^ 


,.  *—  • 


fl 


*t  ■■,'::.;"■, ."-; ,  ■ 


>>!* 


Fig.  4. — French  prune  tree  in  full  bloom,  typical  of  kind  of  trees  used  in  the 

experiment. 


ing  the  work  of  1916,  a  number  of  prune  growers  in  the  Santa  Clara 
Valley  and  in  other  sections  of  the  state  kept  bees  in  their  orchards 
during  the  blossoming  period.  A  brief  summary  of  results  in  each 
case  follows : 


220 


UNIVERSITY   OF    CALIFORNIA EXPERIMENT   STATION 


Mr.  H.  H.  Dingley  of  Saratoga  placed  fifty  hives  of  bees  near  the  edge  of  his 
orchard  containing  about  fifty  acres.  When  this  orchard  was  visited  in  August, 
the  trees  were  literally  breaking  beneath  the  crop.  Mr.  Dingley  also  reports  a 
large  crop  on  some  cherry  trees  nearby. 

Mr.  J.  F.  Pyle  of  San  Jose  kept  a  number  of  hives  of  bees  in  his  orchard. 
This  orchard  was  not  visited,  but  the  owner,  in  a  letter,  said  in  part: 

"Our  Jefferson  plum  orchard  which  has  never  borne  a  fair  crop  has  a  heavy 
crop  this  year  .  .  . ;  in  fact,  all  our  trees  of  all  varieties,  peaches,  apples,  plums  and 
prunes,  have  an  exceptionally  heavy  crop. ' ' 


Fig.  5. — Heavy  crop  on  Tragedy  plum  tree  in  E.  W.  Skinner's  orchard,  Yuba 
City,  Cal.  Eesult  of  keeping  a  large  number  of  colonies  of  bees  in  the  orchard 
during  the  blossoming  period. 


About  fifty  colonies  of  bees  were  kept  in  the  orchard  of  Mr.  C.  E.  Jackson 
of  Santa  Clara.  When  this  orchard  was  inspected,  the  prune  trees  were  loaded 
to  capacity,  and  the  owner  was  also  harvesting  a  full  crop  of  Washington  plums 
from  trees  that  had  never  before  produced  a  satisfactory  yield. 

Mr.  E.  W.  Skinner  of  Yuba  City  placed  113  colonies  of  bees  in  his  mixed 
orchard  of  about  eighty  acres.  All  varieties  of  fruit  set  exceptionally  heavy 
crops,  which  was  attributed  by  the  owner  to  the  work  of  bees  (Fig.  5).  Mr.  Skin- 
ner with  Mr.  H.  P.  Stabler,  Horticultural  Commissioner  of  Sutter  County,  also 
conducted  a  number  of  experiments  to  test  the  work  of  bees  in  the  orchard. 
Several   branches   of   different  kinds   of   fruit   trees   were   covered  with   mosquito 


THE   COMMON   HONEY  BEE  IN  PRUNE  POLLINATION 


221 


bar  so  as  to  exclude  bees  and  similar  insects.  The  result  was  a  heavy  set  of 
fruit  on  all  parts  of  the  several  trees  except  the  branches  covered  by  the  mosquito 
netting.2 

HOW   THE    WORK    WAS    DONE 

The  mosquito-bar  tents  were  erected  over  the  trees  chosen  for  the 
experiment  before  any  of  the  flower  buds  had  opened.  The  frame- 
work was  started  March  12,  1917,  and  the  tents  were  completed  on 
March  15,  1917.      As  the  blossoms  opened  slowly  due  to  the  back- 


Fig.  6. — A  few  of  the  bee  colonies  in  Sorosis  orchard  during  the  blossoming 

period. 


wardness  of  the  season,  it  was  not  until  March  23  that  enough  flowers 
were  sufficiently  advanced  to  justify  placing  the  bees  under  the  tents. 
The  Imperials  had  opened  three  or  four  days  ahead  of  the  French, 
but  there  was  still  a  large  number  of  the  blossoms  of  the  former  open 
when  the  majority  of  the  blossoms  of  the  latter  variety  reached  the 
stage  when  pollination  could  take  place.  The  bee  colonies  were  placed 
under  the  tents  on  the  evening  of  March  23  and  removed  on  the 
evening  of  March  29,  after  having  been  confined  for  six  days.  During 
the  first  day  or  two  the  bees  did  not  do  much  work,  but  occupied  their 


2  A  full  account  of  this  test  is  given  in  the  Pacific  Eural  Press  for  April  7, 
1917. 


222  UNIVERSITY   OF   CALIFORNIA- — EXPERIMENT   STATION 

time  trying  to  escape.  By  March  26,  however,  they  had  settled  down, 
seemed  contented,  and  kept  busily  at  work  on  the  blossoms.  The 
tents  were  removed  April  14  after  all  the  blossoms  had  fallen.  The 
weather  throughout  the  time  the  bees  were  under  the  tent  was  ideal 
for  pollination  purposes,  being  warm  and  clear,  with  little  or  no  wind. 
Approximately  2000  blossoms  were  counted  on  each  of  the  several 
trees  in  the  experiment.  The  branches  on  which  these  blossoms  were 
situated  were  tagged  after  the  method  used  in  pollination  experiments, 
and  the  number  of  fruits  resulting  from  these  blossoms  were  carefully 
watched  throughout  the  season.  From  these  figures  the  percentages 
of  fruit  set  was  obtained.  A  large  number  of  blossoms  were  counted 
on  trees  not  treated  in  any  way  to  obtain  the  average  percentage  of 
set  that  prevailed  in  the  orchard.  The  trees  used  to  obtain  this 
average  orchard  set  were  the  same  ones  used  for  that  purpose  in  1916, 
so  the  average  set  for  the  orchard  for  the  two  years  may  be  studied 
without  regard  to  differences  due  to  individual  tree  variation. 

RESULTS    OF    EXPERIMENTS 

The  results  obtained,  as  in  1916,  were  very  decisive,  and  the 
figures  show  greater  uniformity  than  do  those  obtained  in  the  previous 
season's  work.      Tables  1  and  2  give  the  results  of  the  year's  work. 

TABLE  I 

Behavior  of  French  Prune  Trees  With  and  Without  Cross-pollination, 

Season  of  1917 

Blossoms  Matured  fruits 

counted  counted  Per  cent  of 

Treatment  Mar.  21,  1917       Aug.  7,  1917  fruit  set 

French  prunes:   average  orchard  set 7272  962  13.2 

French  prune  tree :    under   tent  from  which 

bees  were   excluded 1391  6  0.43 

French  prune  tree:    enclosed  with  bees  and 

an    Imperial   tree 2120  330  15.5 

French  prune  tree:  enclosed  with  bees  alone  2152  409  19.0 

The  average  orchard  set  of  the  French  prune  trees  used  to  check 
the  work  with  the  bees  was  13.2  per  cent,  which  is  exceptionally  high 
for  the  variety.  This  was  probably  due  to  the  fact  that  the  trees  were 
near  the  Imperials  and  not  far  removed  from  one  of  the  several 
colonies  of  bees  placed  in  the  orchard  by  the  owner  (Fig.  6). 

With  the  French  prune  tree  from  which  all  pollen-carrying  insects 
were  excluded,  only  0.43  per  cent  of  the  blossoms  set  fruit.  This 
result  illustrates  the  necessity  for  pollen-distributing  agents  like  bees 
in  the  orchard. 


THE   COMMON   HONEY   BEE  IN  PRUNE  POLLINATION  223 

Where  a  French  and  an  Imperial  tree  were  enclosed  in  a  tent 
tog-ether  with  a  colony  of  bees  the  set  of  the  French  tree  was  15.5 
per  cent.  No  doubt,  practically  every  blossom  on  this  tree  was 
visited  one  or  more  times  during  the  period  the  bees  were  in  the  tent. 

The  French  tree  alone  under  a  tent  with  bees  set  19.4  per  cent. 
This  high  set  is  also  probably  due  to  the  fact  that  each  blossom  was 
visited  a  number  of  times. 

The  pronounced  difference  between  the  tree  which  had  no  pollen- 
carrying  agents  at  work  upon  it  and  the  trees  which  were  exposed 
to  bees  is  apparent  at  a  glance. 

TABLE  II 

Behavior   of   Imperial   Prune   Tree   With   and   Without    Cross-pollination, 

Season  of  1917 

Blossoms  Matured  fruits 

counted  counted  Per  cent  of 

Treatment  Mar.  21,  1917       Aug.  7,  1917  fruit  set 

Imperial  prunes:    average   orchard  set 1535  112  7.2 

Imperial  prune  tree:  under  tent  from  which 

bees  were   excluded .• 2036  7  0.34 

Imperial  prune  tree:  enclosed  with  bees  and 

a   French   tree 2153  170  7.9 

Imperial    prune    tree:     enclosed    with    bees 

alone    2184  66  3.02 


The  average  orchard  set  of  the  Imperial  prune  trees  was  7.2  per 
cent.  This  percentage  represented  a  good  crop,  when  the  profusion 
of  blossoms  on  each  tree  is  taken  into  consideration. 

The  set  of  the  Imperial  tree  from  which  all  pollen-carrying  insects 
were  excluded  was  only  0.34  per  cent.  Here  again,  the  need  of  a 
pollen-distributing  agency  is  plainly  indicated. 

When  enclosed  with  a  colony  of  bees  together  with  a  French  tree, 
the  Imperial  tree  set  7.9  per  cent.  This  result  shows  that  most  of 
the  flowers  had  been  visited  by  the  bees,  and  many  of  them  had  been 
caused  to  set  fruit. 

The  Imperial  tree  upon  which  the  bees  worked  without  coming 
into  contact  with  the  blossoms  of  any  other  tree  showed  a  set  of  3.02 
per  cent.  The  persistent  work  of  the  bees  which  visited  every  flower 
was  probably  responsible  for  this  decisive  percentage  of  set.  As 
was  the  case  of  the  French,  the  value  of  bees  for  pollination  purposes 
with  the  Imperial  is  strikingly  shown. 

The  results  of  the  work  during  the  season  of  1917  agree  in  general 
with  the  important  results  obtained  in  1916.  Figures  7  and  8  present 
a  graphic  comparison  of  the  season's  work. 


224  UNIVERSITY   OF    CALIFORNIA EXPERIMENT   STATION 

Where  the  French  and  Imperial  prune  trees  were  enclosed  in  the 
tent  with  bees  the  set  of  fruit  was  greater  with  the  French  variety 
than  the  average  set  for  the  orchard.  The  increase  in  the  percentage 
of  set  was  undoubtedly  due  to  the  work  of  the  bees. 

During  both  years,  the  French  trees  from  which  all  bees  and 
similar  insects  were  excluded  set  a  negligible  crop.  Without  insect 
aid,  these  trees  were  unable  to  set  fruit. 

In  1917  the  French  tree  which  alone  was  enclosed  in  a  tent  with  a 

French  prunes 


/f/7  XZE2ZZZZZ&'3-*% 

/<?/£>  I   /<o<?fc.      j^f-   Qn    rr<-€><i      -from*    yyr/,/c/f 

/f/7  I  0>f3%      6***      vver*       * x<l /</ </<- c/. 

/W  WBtKUBBUA   S.O o~  *;     &--?-  o^    Tree?* 

~  — „       <?S7c  /o<&<s.c/    wfff  ^cj- 

/f/7  V/////M///M  sttZ      v*v/tf      Jhtfmr/**: 


Get"  on  //*ee    &/7c/o$ec/ 

/f/7     W2ZZZZZ5&Z&&  /*«*    W,M     ^ee*      c/one. 

Fig.  7. — Effect  of  the  work  of  bees  on  the  set  of  French  prunes  during  the  seasons 

of  1916  and  1917. 


colony  of  bees,  set  a  crop  of  19.4  per  cent.  This  test  was  not  made 
in  1916. 

The  unexplained  results  of  the  "lower  than  average''  percentage 
of  set  in  1916  on  the  Imperial  tree  which  was  enclosed  in  a  tent  with 
a  colony  of  bees,  together  with  a  French  tree,  were  not  duplicated. 
Instead,  an  increase  over  the  average  orchard  set  was  obtained  when 
this  combination  was  used  in  1917.  The  tree  used  in  1917  responded 
to  the  work  of  the  bees  in  a  normal  manner. 

The  Imperial  trees  from  which  pollen-carrying  insects  were 
excluded  gave  a  consistently  low  percentage  of  set,  as  shown  by  0.0 
per  cent  in  1916  and  0.34  per  cent  in  1917.  No  crop  was  produced 
in  the  absence  of  bees,  in  spite  of  the  great  abundance  of  flowers. 


THE   COMMON   HONEY   BEE  IN  PRUNE  POLLINATION  225 

The  Imperial  tree  enclosed  in  a  tent  alone  with  a  colony  of  bees 
set  a  fair  crop.      This  test  was  not  tried  in  1916. 

The  percentage  of  set  of  French  primes  under  open  orchard  con- 
ditions during  both  seasons  shows  what  may  be  accomplished  by  the 
use  of  bees  in  the  orchard.  In  1916  there  were  six  colonies  of  bees 
to  180  acres  of  prunes,  and  the  resultant  average  set  was  3.6  per  cent 
(Fig.  7).  In  1917,  with  115  colonies  of  bees  in  the  same  orchard,  the 
set  was  13.2  per  cent,  an  increase  of  9.6  per  cent  apparently  due  to 
the  work  of  bees. 


J./77/^c.r/c//  ^A^ej 


/<?/7     1&yZ&?Z7o  y  ^"<"~c      oca 

'<?/£  O^  v5e/    0^7     7>-e**     from    Vv/tvc/* 

Sf'7    V7777Z79K     ""**  ****    wS/t    /^n<z/f 


f«7  jb***x  7C:z^xfjc/~^^ 

Fig.   8. — Effect  of  the  work  of  bees   on  the  set  of  Imperial  prunes   during  the 

seasons  of  1916  and  1917. 

The  Imperial  trees  did  not  respond  to  the  work  of  bees  in  the  same 
way  as  did  the  French.  The  average  orchard  set  for  this  variety 
for  both  years  was  7.2  per  cent.  These  yields,  however,  were  con- 
sidered to  be  exceptionally  good  for  the  variety. 


BEHAVIOR  OF  TREES   UNDER   EXPERIMENT   IN    1916   BUT   LEFT   UNDER 
OPEN    ORCHARD    CONDITIONS    IN    1917 

The  trees  experimented  upon  in  1916  and  caused  to  bear  either 
an  exceptionally  heavy  crop  or  a  very  light  one,  were  closely  watched 
during  the  season  of  1917,  when  they  were  left  under  open  orchard 


226  UNIVERSITY   OF    CALIFORNIA EXPERIMENT   STATION 

conditions.  The  French  tree  which  had  produced  a  very  heavy  crop 
of  fruit  in  1916  due  to  the  work  of  bees,  bloomed  later  than  the  rest 
of  the  trees  of  that  variety,  and  produced  only  about  one-fourth  as 
many  blossoms  as  did  trees  of  similar  size  (Fig.  9) .  The  weight  of  the 
crop  borne  the  previous  season  was  still  shown  by  the  branches,  many 
of  which  were  nearly  horizontal.  The  tree  which  had  borne  but  a 
very  small  crop  produced  an  abundance  of  blossoms  at  the  normal 
time  (Fig.  10).  These  trees  also  showed  differences  in  the  amount  of 
fruit  produced.      The  results  are  shown  in  Table  3. 

The  average  set  of  the  orchard  for  French  prunes  increased  from 
3.6  per  cent  in  1916  to  13.2  per  cent  in  1917,  as  was  noted  previously. 
The  figures  are  given  here  to  afford  a  comparison  with  the  trees  in 
the  experiment. 

The  French  prune  tree  which  in  1916  set  only  1.04  per  cent  because 
the  bees  were  kept  away  from  it,  this  year  set  10.1  per  cent  under 
open  orchard  conditions.  This  set,  although  not  as  heavy  as  the 
average  for  the  orchard,  was  considerably  larger  than  the  crop  pro- 
duced on  the  tree  which  had  carried  the  large  crop  during  the  previous 
season.  This  response  was  probably  due  to  the  fact  that  the  tree 
was  not  forced  to  devote  all  of  its  energy  to  maturing  a  fruit  crop, 
but  was  able  to  produce  fruit-bearing  wood  for  the  following  season. 

On  the  other  hand,  the  French  tree  which  in  1916  set  a  heavy 
crop  (18.05  per  cent)  as  the  result  of  the  work  of  bees,  dropped  to 
7.4  per  cent,  or  about  one-half  of  the  average  set  of  the  orchard. 
This  drop  was  to  be  expected,  because  of  the  strain  put  upon  the  tree 
in  maturing  an  abnormally  large  crop. 

The  record  of  both  Imperial  trees  used  in  the  experiment  in  1916 
showed  a  very  light  yield  for  that  year.  The  tree  from  which  bees 
were  excluded  set  no  fruit,  while  the  other  tree  unexplainedly  set 
only  1.7  per  cent,  as  was  mentioned  in  a  previous  report.  In  1917 
these  trees  set  9.0  per  cent  and  10.7  per  cent,  respectively,  which  is 
higher  than  the  average  in  both  cases.  This  increase  was  probably 
due  to  the  fact  that  both  trees  were  able  to  produce  new  fruit  wood 
and  strengthen  the  older  parts.  The  results  for  both  varieties  are 
shown  graphically  in  figure  11. 

In  addition  to  the  results  already  given,  counts  have  been  made 
for  three  years  on  French  prunes  under  open  orchard  conditions  at 
different  distances  from  the  Imperials.  One  set  of  counts  was  made 
on  a  row  of  French  adjoining  the  Imperials,  and  another  was  made 
on  a  row  of  French  five  rows  from  the  nearest  Imperial  row.  The 
results  show  in  each  case  a  slightly  larger  percentage  of  set  on  the 
French  trees  next  to  the  Imperials  than  on  those  farther  away.      In 


THE   COMMON   HONEY   BEE  IN  PRUNE  POLLINATION 


227 


SI 


W^Bm*'       :±J^  * ^■^^^^wMs^^efBH^^^s^B^m>^^i^y^--  ■■"■v-^^^^m^^m^^.t 


Fig.  9. — French  prune  tree  showing  drooping  branches  and  scarcity  of  blossoms 
as  a  result  of  the  abnormally  large  crop  produced  the  previous  season.  Trees  in 
background  are   in   full  bloom.       Compare  with  fig.   10. 


228 


UNIVERSITY   OF    CALIFORNIA EXPERIMENT   STATION 


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THE   COMMON   HONEY  BEE  IN  PRUNE  POLLINATION 


229 


. 


h      * •    4  ■      '--Vt  >  s&$f  x  '  *£^*#'Sr  fl-f  ■  ■  ■"■X.  /  -'  -^r 


•*. 


****** 


Fig.  10. — French  prune  tree  caused  to  produce  a  very  light  crop  the  previous 
season  by  the  enforced  exclusion  of  pollen-carrying  insects.  Note  the  abundance 
of  bloom.      Compare  with  fig.  9. 


230  UNIVERSITY   OF    CALIFORNIA EXPERIMENT   STATION 

1915  the  French  set  5.5  per  cent  on  trees  adjoining  the  Imperials  and 
2.6  per  cent  on  trees  five  rows  away.  In  1916  the  results  were  3.6 
per  cent  and  3.1  per  cent  respectively.  The  decisive  difference 
between  15.1  per  cent  and  10.8  per  cent  was  obtained  in  1917  when 
counts  were  made  on  the  same  trees  as  in  the  previous  season. 

DISCUSSION   OF   RESULTS 

The  work  of  the  year  just  passed  again  shows  emphatically  the 
value  of  bees  as  pollen-carrying  agents.  Reports  from  a  number  of 
growers  who  kept  bees  in  their  orchards  during  the  blossoming  season, 
agree  with  the  results  obtained  experimentally  on  the  Sorosis  Ranch. 
In  every  case  a  substantial  increase  in  the  size  of  the  crop  was  re- 
ported. While  no  experimental  counts  were  made  in  these  orchards, 
an  inspection  of  them  substantiated  the  claims  put  forth  by  the 
owners. 

At  the  Sorosis  Ranch,  the  increase  on  the  French  trees  due  to  the 
use  of  bees  was  remarkable.  These  insects  are  at  least  partly  re- 
sponsible for  the  increase  from  3.6  per  cent  in  1916,  when  practically 
no  bees  were  present,  to  13.2  per  cent  when  there  were  115  colonies 
in  a  180-acre  orchard  (Fig.  7).  The  value  of  these  insects  is  further 
shown  by  the  set  of  15.5  per  cent  on  French  trees  adjoining  the 
Imperials  and  incidentally  closer  to  a  number  of  colonies  of  bees,  as 
compared  with  the  set  of  10.8  per  cent  five  rows  farther  away  from  the 
Imperials  and  from  the  bees. 

Even  more  striking  results  were  obtained  on  the  French  trees 
which  were  under  the  tents.  Here  the  bees  and  similar  insects  were 
entirely  excluded  or  confined,  and  in  the  latter  case  forced  to  work 
several  days  on  only  the  blossoms  of  the  trees  under  experiment. 
The  trees  from  which  the  bees  were  excluded,  though  given  the  same 
cultural  treatment  as  the  check  trees,  failed  to  set  even  a  fair  crop. 
The  data  from  both  years  point  to  the  necessity  of  some  agency  for 
distributing  pollen  and  show  the  beneficial  results  from  the  use  of 
such  an  agency.  The  trees  themselves  produced  a  vigorous  new 
wood  growth  (Fig.  12).  On  the  other  hand,  by  confining  bees  to 
two  trees  the  percentage  of  set  was  greatly  increased.  Although  the 
marked  increase  obtained  in  the  experiment  could  not  be  obtained 
in  a  commercial  orchard  because  of  the  impossibility  of  confining 
bees  to  a  certain  section,  still  a  noticeable  benefit  should  result  from 
the  keeping  of  bees  in  conjunction  with  the  prune  orchard. 

In  addition  to  the  data  which  show  that  the  French  prune  can 
be  cross-pollinated  with  the  Imperial,  the  former  variety  is  also  appar- 
ently self -fertile.      Under  a  tent  enclosing  a  single  French  tree  with 


THE   COMMON  HONEY   BEE   IN  PRUNE  POLLINATION  231 

a  colony  of  bees,  the  percentage  of  set  on  the  tree  was  higher  than 
the  average  for  the  orchard,  and  also  higher  than  the  set  on  the  tree 
where  cross-pollination  with  the  Imperial  was  possible.  The  19.4 
per  cent  set  obtained  in  this  case  would  clearly  seem  to  indicate  that 

/^r^nc/f   Prunes 


/?/7   Y/////A  h.Vtf    <>efor?c/er  OrcAart/  co/7</,f/on$ 

~/77/)e>r/a/  firun^s. 

ZZ.  <7*  Average      o/-c/iarc/    <Se7^ 

79/7  Y////AA*9.o%  vet  ur?</er    ore /? art/   conc/zfyan  <>. 

W6   ■  )/?#    Set  cnc/oscc/    *s/M      6<s*S. 

/f/7  WS/////SA) /<>./%  &**    unJer    Orc/harJ  cono'fr/'ons. 

Fig.  11. — Production  of  trees  caused  to  bear  heavy  or  light  crops  in  1916,  com- 
pared with  the  production  on  the  same  trees  in  1917. 

this  variety  was  amply  able  to  set  fruit  with  its  own  pollen,  providing 
the  pollen  was  properly  distributed.  The  difference  in  percentage  of 
set  between  the  French  alone  and  the  one  enclosed  with  the  Imperial  is 
but  3.9  per  cent.  It  is  not  improbable  that  this  difference  is  due 
to  individual  tree  variation.     Neither  can  these  results  be  construed 


/9/t  BBBH 

aeaa^^i 

232 


UNIVERSITY   OF    CALIFORNIA EXPERIMENT   STATION 


to  mean  that  it  is  not  beneficial  to  plant  Imperials  among  the  French. 
On  the  contrary,  the  results  of  three  years  show  a  higher  percentage 
of  set  on  the  French  trees  close  to  Imperials  than  on  those  farther 
away.  The  large  yield  obtained  by  using  bees  on  a  single  French 
tree,  showing  a  degree  of  self-fertility,  may  throw  some  light  on 
the  question  why  the  prune  orchards  of  the  state  which  are  largely 
of  this  variety  have  been  able  to  produce  fairly  satisfactory  crops 
year  after  year. 


:"^£ 


■"2    f  .:"■     '.'  V 


rtm'*taa? 


Fig.  12. — French  prune  tree  from  which  all  pollen-carrying  insects  were 
excluded.  It  bore  practically  no  fruit  but  produced  a  vigorous  new  wood 
growth  as  shown  by  the  shoots  near  the  top  of  the  tree. 


With  the  Imperials,  the  factor  of  the  condition  of  the  trees  in 
all  probability  had  some  influence  on  the  percentage  of  fruit  set 
(Fig.  13).  These  trees  were  top-worked  on  Robe  de  Sergeant  in 
1912.  In  many  cases  the  wounds  made  in  grafting  permitted  the 
entrance  of  heart-rot  fungi,  with  the  result  that  many  branches  died 
or  broke  down  each  year  (Fig.  13).  Although  apparently  vigorous, 
many  of  them  seemed  to  be  incapable  of  response  to  the  stimulation 
of  increased  pollination.  It  is  noteworthy  that  the  percentage  of 
set  was  maintained  at  the  high  figure  of  7.2. 


THE   COMMON   HONEY   BEE  IN  PRUNE  POLLINATION 


233 


The  Imperial  trees  kept  under  tents  during  the  blossoming  season 
also  showed  decisive  results,  although  somewhat  less  pronounced  than 
those  obtained  with  the  French.  With  this  variety  the  lack  of  proper 
pollen  distribution  was  shown  by  the  lack  of  fruit  in  1916  and  the 
very  small  (0.43  per  cent)  set  in  1917,  when  all  pollen-carrying 
agencies  were  excluded. 

When  enclosed  in  a  tent  with  a  French  tree  and  a  colony  of  bees, 
the  Imperial  tree  set  7.9  per  cent,  which  was  0.7  per  cent  above  the 


Fig.   13. — Imperial  prune  tree   broken   down  under  heavy  load   due   to   heart-rot 
fungi  which  gained  entrance   through  large   grafting  wounds. 


average  for  the  orchard.  This  small  increase  may  have  been  due 
to  the  condition  of  the  tree.  While  still  sound  to  the  casual  observer, 
there  was  evidence  of  heart-rot  present.  Although  no  individual  tree 
records  had  been  kept  during  the  previous  years,  the  group  in  which 
this  tree  stood  had,  according  to  the  owner,  been  producing  heavily, 
and  therefore,  it  might  not  have  been  able  to  respond  as  fully  as 
other  trees  in  the  orchard.  Nevertheless  the  increase,  though  slight, 
was  consistent  with  the  results  obtained  with  the  French  variety.  It 
showed,  furthermore,  that  the  results  obtained  in  a  similar  experiment 


234  UNIVERSITY  OF   CALIFORNIA EXPERIMENT   STATION 

in  1916,  when  the  Imperial  under  conditions  identical  with  those  de- 
scribed, set  a  much  smaller  percentage  of  fruit,  were  unusual  and 
would  not  necessarily  be  a  frequent  occurrence  under  commercial 
orchard  conditions.  In  other  words,  an  Imperial  orchard  might 
ordinarily  be  expected  to  set  a  higher  percentage  of  fruit  when  a 
large  number  of  bees  were  present  than  when  they  were  not. 

The  surprising  feature  of  the  work  of  the  1917  season  was  the 
set  of  3.02  per  cent  obtained  on  an  Imperial  tree  enclosed  under 
a  tent  alone  with  a  colony  of  bees.  Results  obtained  up  to  the  present 
season,  in  another  part  of  the  pollination  project  carried  on  by  this 
Division,  had  indicated  that  the  Imperial  was  unable  to  set  fruit  with 
its  own  pollen.  In  this  case,  however,  after  the  bees  had  thoroughly 
worked  on  the  blossoms  for  six  days,  enough  blossoms  were  stimulated 
into  bearing  fruit  to  set  a  satisfactory  commercial  crop.  It  is  pos- 
sible that  the  Imperial,  though  usually  self-sterile,  in  occasional 
years  at  least  is  partially  self-fertile.  It  is  probable  that  this  self- 
fertility  does  not  occur  very  frequently.  Similar  conditions  are 
found  in  the  case  of  the  apple.3 

The  behavior  of  the  trees  experimented  upon  in  1916  presented 
several  interesting  features  the  past  season.  The  French  tree  which 
in  1916  was  induced  to  bear  an  abnormally  heavy  crop,  this  year  set 
a  light  one.  Moreover,  this  difference  between  18.05  per  cent  and 
7.4  per  cent  does  not  tell  the  entire  story.  In  1917  the  tree  not 
only  had  fewer  blossoms  but  actually  set  a  lower  percentage  of  this 
reduced  number  of  blossoms.  Incidentally  the  blossoms  appeared 
about  a  week  later  than  the  average.  They  were  also  fewer  in  num- 
ber and  apparently  weaker  both  in  general  appearance  and  in 
performance.  The  entire  tree  showed  the  effects  of  the  previous 
season's  overload  and  made  but  little  vegetative  growth.  The  per- 
formance of  this  individual  tree  may  be  taken  as  an  example  of  what 
may  possibly  happen  in  an  entire  orchard,  to  a  greater  or  less  degree, 
if  the  trees  are  overloaded  by  suddenly  increased  pollination.  This 
condition  may  also  last  until  the  trees  become  adapted  to  the  changed 
factor  of  pollen  distribution.  However,  it  is  not  likely  that  enough 
bees  could  be  put  in  the  orchard  to  duplicate  the  conditions  under 
the  tents  in  the  experiments  just  described. 

Under  the  opposite  condition  where  bees  were  kept  from  the 
French  tree  and  a  very  small  crop  was  set  (Fig.  11)  in  1916,  the 
yield  in  1917  showed  a  percentage  of  set  of  10.1  per  cent.     This  tree 

s  According  to  E.  J.  Kraus  of  the  Oregon  Experiment  Station,  ' '  some  varieties 
of  the  apple  have  been  found  to  be  self -sterile  three  years  out  of  five  and  self- 
fertile  the  other  two." 


THE   COMMON  HONEY  BEE  IN  PRUNE  POLLINATION  235 

behaved  normally  in  every  respect.  It  produced  an  abundance  of 
flowers  and  set  an  entirely  satisfactory  crop.  Its  performance  may 
be  likened  to  what  would  occur  in  case  a  number  of  colonies  of  bees 
were  placed  in  an  orchard  that  without  bees  had  only  produced  small 
crops. 

Both  of  the  Imperial  trees  used  in  the  experiments  in  1916  set 
but  very  small  crops  that  year  (Fig.  11).  When  exposed  to  open 
orchard  conditions  in  1917,  they  both  responded  by  producing  crops 
that  were  above  the  average  for  the  variety.  The  Imperial  was  thus 
also  shown  to  be  capable  of  producing  larger  crops  when  the  number 
of  pollen-carrying  insects  was  increased  The  trees  which  bore  no 
crop  in  1916  showed  a  larger  wood  growth  than  the  average  for  the 
variety. 

SUMMARY 

The  summary  of  the  year 's  work  is  as  follows : 

1.  The  average  orchard  set  of  French  prunes  was  increased  by  the 
use  of  bees  in  the  orchard. 

2.  No  increase  in  the  average  orchard  set  of  the  Imperials  resulted 
when  bees  were  used  in  the  orchard. 

3.  The  percentage  of  set  in  both  French  and  Imperial  prunes  was 
greatly  reduced  when  all  pollen-carrying  insects  were  kept  away  from 
the  trees. 

4.  The  percentage  of  set  on  the  French  tree  enclosed  in  a  tent  with 
an  Imperial  tree  and  a  colony  of  bees  was  higher  than  the  orchard 
average  for  the  variety. 

5.  The  French  tree  enclosed  alone  under  a  tent  with  bees  also  set 
a  higher  percentage  of  fruit  than  the  average  for  the  variety. 

6.  The  percentage  of  set  on  the  Imperial  tree  enclosed  in  a  tent 
with  a  French  tree  and  a  colony  of  bees  was  slightly  higher  than  the 
average  for  the  variety. 

7.  A  fair  crop  of  fruit  was  set  on  the  Imperial  tree  enclosed  alone 
under  a  tent  with  bees. 

8.  The  French  tree  caused  to  bear  a  heavy  crop  in  1916  by  the  use 
of  bees  set  a  very  small  crop  in  1917. 

9.  The  French  tree  caused  to  bear  a  light  crop  in  1916  by  the 
exclusion  of  bees,  set  a  satisfactory  percentage  of  fruit,  although  not 
as  high  as  the  orchard  average,  in  1917. 

10.  Both  Imperial  trees  which  produced  light  crops  in  1916,  set 
heavy  crops  in  1917  when  under  open  orchard  conditions. 

11.  The  percentage  of  set  on  the  French  trees  was  inversely  pro- 
portional to  the  distance  of  the  French  from  the  Imperials. 


236  UNIVERSITY   OF    CALIFORNIA EXPERIMENT   STATION 

CONCLUSIONS 

The  results  of  the  two  seasons'  work  seem  to  warrant  the  following 
conclusions : 

1.  Both  the  French  and  Imperial  prunes  may  be  aided  in  setting 
fruit  by  the  use  of  bees  in  the  orchard  during  the  blossoming  period, 
provided  the  trees  are  in  a  normal,  healthy  condition. 

2.  The  absence  of  bees  in  the  orchard  may  mean  a  low  percentage 
of  set  with  both  of  these  varieties. 

3.  The  French  prune  does  not  absolutely  require  interplanting 
with  the  Imperial,  even  though  this  arrangement  may  prove  beneficial 
to  both  varieties. 


